Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

849
FXUS63 KLOT 181101
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
601 AM CDT Thu Sep 18 2025

.KEY MESSAGES...

- Patchy fog remains possible early this morning.

- Warm and primarily dry conditions persist through Friday afternoon.

- Periodic chances for showers and some thunderstorms will
  return Friday night into next week, though there will continue
  to be plenty of dry hours too.

&&

.DISCUSSION...
Issued at 355 AM CDT Thu Sep 18 2025

Through Friday...

The ridge of high pressure that has brought us the sunny and
warm weather the past several days remains overhead, but is
starting to be impeded by the broad trough (and its associated
shortwaves) centered over the north-central CONUS. That said,
our tranquil weather should hold through Friday afternoon as the
ridge slowly exits to the east. So expect a couple more warm
afternoons with highs in the mid to upper 80s inland and mid to
upper 70s near the lake due to onshore winds. In fact, the lake
breeze today will actually be added by a backdoor cold front
that will surge down the lake this afternoon which will push
cooler temperatures further inland this evening across northeast
IL.

However, there is a small chance (around 20-25%) for some
scattered showers and thunderstorms to develop near and west of
I-39 Friday afternoon as the shortwave over KS ejects into the
Mississippi River Valley. Given that forecast soundings still
show a bit of dry air to contend with, suspect most areas could
still remain dry.

But before all of this gets underway there is some patchy fog
to deal with first. Similar to last night, the combination of
clear skies and light winds has allowed temperatures to cool
near the dew points and give way to some patchy fog particularly
in far northeast IL, the Fox Valley, and northwest IN. While
some expansion of the fog is possible prior to daybreak, suspect
visibilities for most will be in the 3-5 mile range. Though
locally denser pockets of fog are possible especially closer to
the lake and northwest IN where a reservoir of higher dew points
resides. Any fog that does develop will erode by 8-9 AM this
morning.

Friday night through Wednesday...

The aforementioned trough and the associated developing surface
low will continue to move east into the upper Midwest and
northern Great Lakes Friday night into Saturday. This will force
a frontal boundary eastward across northeast IL and northwest
IN Friday night which in turn will push any shower/storms that
develop Friday afternoon near I-39 eastward. While this front
will advect some better moisture into the area, there is still
forecast to be a fair amount of dry air across the eastern 1/3rd
of the area which may limit shower and thunderstorm coverage,
particularly for the Chicago metro and northwest IN. Regardless
of the shower/storm coverage Friday night, the broad forcing
from the trough just to our north and the stalled frontal
boundary should still be sufficient to kickoff periods of
scattered showers and thunderstorms Saturday and again on
Sunday. Though, plenty of dry hours can be expected too given
the more modest instability which may focus showers and storms
mainly to the afternoon and early evening hours.

A more amplified shortwave trough is expected to eject out of
the northern Plains on Monday and pivot across the Upper
Midwest/northern Great Lakes Monday night. Associated with this
wave will be a more formidable cold front which is forecast to
swing across our area late Sunday and through the day on Monday
brining with it a much broader coverage of showers and
thunderstorms. As expected, rain chances for the Sunday night-
Monday timeframe have continued to increase and look to be the
highest of the weekend at 50-70%. That said, wind shear values
and PWATs this weekend through Monday continue to look rather
modest so the threat for any severe weather and/or heavy
rainfall/flooding is still low. Outside of the rain,
temperatures for this weekend and Monday continue to look
seasonably warm with readings in the upper 70s to lower 80s each
afternoon.

From Tuesday onward, the forecast is still very uncertain.
While guidance has started to trend towards a broad
trough/closed upper low pivoting into the mid-Mississippi Valley
Tuesday into Wednesday, there continues to be differences on
timing and longevity of this system over our area. If the
shorter duration solutions play out then we could be looking at
another glancing blow of showers and storms through midweek
followed by a period of dry weather. However, if the longer
duration solutions pan out then we could be looking at shower
and storm chances hanging around for much of next week. Given
the uncertainly, saw no need to adjust the 20% POPs offered by
the blended forecast for this period but will continue to
monitor trends over the coming days. Regardless of how the rain
plays out next week, temperatures are forecast to favor near to
slightly above average for late September due to persistent
northwest flow aloft.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 600 AM CDT Thu Sep 18 2025

A brief dip in VSBYs to near MVFR (BR) will remain possible at
area terminals around sunrise, otherwise VFR conditions are
forecast through the period.

Light and variable winds will continue through the morning
hours with a lake breeze expected to push inland by early
afternoon turning winds east to northeast around 5-10 kt.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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